1. Field of the Invention
This invention relates to a magnetic disk cartridge. This invention particularly relates to a magnetic disk cartridge comprising a small-diameter, very thin magnetic disk which is rotated at a high speed for recording and reproducing of image information or the like.
2. Description of the Prior Art
Floppy disks are widely used as recording media for computers because they are easy to handle and low in cost. The floppy disk comprises a disk-like base made of a flexible polyester sheet or the like, and magnetic material layers overlaid on opposite surfaces of the disk-like base. The floppy disk is rotated at a high speed for magnetically recording information in the magnetic material layers by use of a magnetic head.
On the other hand, by utilizing the aforesaid advantages of the floppy disk and the advantage that a magnetic recording medium has over a silver halide photographic film in that the magnetic recording medium can be reused, it has been proposed to use a magnetic disk having a size smaller than the size of the floppy disk as an image recording medium in still cameras. The magnetic disk used for still cameras comprises a small-diameter, very thin magnetic recording medium, and a hub secured to the center of the magnetic recording medium. The magnetic disk is rotatably housed in a small, hard case provided at the center thereof with a hub hole for exposing the hub of the magnetic disk. The case also has magnetic head insertion apertures for exposing a part of the magnetic disk in the front surface and the rear surface of the case. The combination of the magnetic disk with the case is called a magnetic disk cartridge.
In the magnetic disk cartridge of the aforesaid type, it sometimes occurs that information stored in the magnetic recording medium is erased by an erroneous operation of the recording and reproducing apparatus. Therefore, it is desired to provide the magnetic disk cartridge with a means for preventing such erroneous erasure. For this purpose, there has heretofore been used a lug which is fabricated integrally with the case of the magnetic disk cartridge and which is broken by the user when necessary to prevent erroneous erasure. In this method, however, when information has to be recorded in the magnetic recording medium of the magnetic disk cartridge after the lug of the magnetic disk cartridge is broken, it is necessary to conduct a troublesome operation such as closing of the lug hole with cellophane adhesive tape. As a means for preventing erroneous erasure, which makes it possible to easily restore the magnetic disk cartridge to the condition in which recording is possible after it has once been put in the condition to prevent erroneous erasure, it has been proposed to employ a plug for insertion into and ejection from the case. However, the magnetic disk cartridge of the type used in a still camera is loaded into a camera body having approximately the same size as that of a 35 mm camera. Thus the magnetic disk cartridge is fabricated to have a very small size and a very small thickness. Therefore, when the aforesaid plug is used for preventing erroneous erasure, the magnetic disk cartridge becomes very difficult to operate since, for example, a small, special pin is required to insert and eject the plug.
FIG. 1 shows another conventional means for preventing erroneous erasure. A slide valve 2 is slidably supported in a case 1, and a detection hole (not shown) closed by the slide valve 2 when the slide valve 2 comes to a predetermined sliding position is positioned in one part of the case 1. A lock arm 2a is fabricated integrally with one side face of the body of the slide valve 2 so that the lock arm 2a can be resiliently deformed approximately in parallel to the body of the slide valve 2. A projection 2b is positioned at one end of the lock arm 2a, and recess portions 1a and 1b for receiving the projection 2b are positioned integrally with the case 1. When the slide valve 2 is moved to the position for closing the detection hole or to the position for opening the detection hole, the projection 2b is supported and stopped in one of the recess portions 1a and 1b. The engagement means constructed as described above operates smoothly at the time the projection 2 b of the resilient lock arm 2a is pulled and disengaged from the recess portion 1a or 1b. However, at the time the projection 2b is fitted into the recess portion 1a or 1b, the lock arm 2a supporting the projection 2b is liable to be deformed or broken, and the engagement means does not operate smoothly.